Inflammation and Wound Healing: the Role of the Macrophage

expert reviews http://www.expertreviews.org/ in molecular medicine Inflammation and wound healing: the role of the macrophage

Timothy J. Koh1 and Luisa Ann DiPietro2,* The macrophage is a prominent inflammatory cell in wounds, but its role in healing remains incompletely understood. Macrophages have many functions in wounds, including host defence, the promotion and resolution of inflammation, the removal of apoptotic cells, and the support of cell proliferation and tissue restoration following injury. Recent studies suggest that macrophages exist in several different phenotypic states within the healing wound and that the influence of these cells on each stage of repair varies with the specific phenotype. Although the macrophage is beneficial to the repair of normally healing wounds, this pleotropic cell type may promote excessive inflammation or fibrosis under certain circumstances. Emerging evidence suggests that macrophage dysfunction is a component of the pathogenesis of nonhealing and poorly healing wounds. As a result of advances in the understanding of this multifunctional cell, the macrophage continues to be an attractive therapeutic target, both to reduce fibrosis and scarring, and to improve healing of chronic wounds.

The inflammatory response following tissue injury molecular pattern molecules (DAMPs) (Ref. 1). has important roles in both normal and The hypoxic environment of the wound also pathological healing. Immediately after injury, promotes inflammation, because hypoxia the innate immune system is activated, setting in stimulates numerous cell types, including motion a local inflammatory response that macrophages, to produce mediators that are includes the recruitment of inflammatory cells important for inflammation (Ref. 2). In response from the circulation. This rapid response begins to these many signals, the levels of leukocyte with the degranulation of platelets that arrive at chemoattractants increase substantially, further the site and the injury-induced degranulation enhancing leukocyte recruitment. of resident mast cells. Local immune cells, As the recruitment of leukocytes from the including resident macrophages, are activated circulation begins in earnest, a pattern of by proinflammatory mediators released in leucocytic infiltration into the wound develops Inflammation and wound healing: the role of the macrophage response to injury, as well as damage-associated that is similar to other acute inflammatory

1 Department of Kinesiology & Nutrition, College of Applied Health Sciences, Center for Wound Healing and Tissue Regeneration, Chicago IL, USA. 2 Department of Periodontics, College of Dentistry, Center for Wound Healing and Tissue Regeneration, University of Illinois at Chicago, Chicago IL, USA. *Corresponding author: Luisa Ann DiPietro, Center for Wound Healing and Tissue Regeneration, MC 859, University of Illinois at Chicago, 801 S. Paulina, Chicago, IL 60612, USA. E-mail: [email protected] 1 Accession information: doi:10.1017/S1462399411001943; Vol. 13; e23; July 2011 © Cambridge University Press 2011

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conditions (Fig. 1). Neutrophils, the most macrophages in wound healing. Two separate abundant white cell in the circulation, infiltrate groups have used murine strains bearing the wound quickly and are the dominant macrophage-restricted expression of the human leukocyte in the earliest stages (Ref. 3). receptor for diphtheria toxin to effect a toxin- Concomitantly with the influx of neutrophils, mediated selective depletion of macrophages circulating monocytes enter the wound and before the placement of wounds (Refs 13, 14). differentiate into mature tissue macrophages The wounds of mice depleted of macrophages in (Ref. 3). Mast cell numbers in the wound also this manner exhibited delayed wound closure, increase, with most of the infiltrating mast cells decreased granulation tissue formation and originating in the adjacent tissue (Ref. 4). In the angiogenesis, decreased collagen synthesis, and late inflammatory phase of wound repair, T cells decreased levels of growth factors, including appear in the wound bed, and may influence vascular endothelial growth factor (VEGF) and the resolution and remodelling of the wound transforming growth factor-β (TGF-β). In (Refs 5, 6). As inflammation resolves and the addition, depletion of macrophages resulted in number of leukocytes diminishes, the wound reduced levels of myofibroblasts, which are undergoes a lengthy period of remodelling and contractile cell types that are important for resolution. Although inflammation is not wound closure. prominent during this resolution phase, many More recently, another group, again using a studies suggest that the events of the diphtheria toxin system, undertook a temporal inflammatory phase have profound effects on selective depletion of macrophages at sequential the final wound outcome (Refs 7, 8). Studies in times during the healing process (Ref. 15). The many different anatomical systems suggest that depletion of macrophages during the early scar formation and fibrosis may derive from inflammatory phase resulted in impaired wound inflammatory cell activity (Ref. 8). closure and granulation tissue formation. Among immune cells in the wound, the role of Depletion during the early proliferative stage the macrophage has been the subject of intensive caused severe haemorrhage and curbed later investigation, yielding more than 600 published wound closure and tissue maturation. articles on the topic within the past 5 years. The Macrophage depletion that was limited to the emerging picture demonstrates that wound tissue maturation phase had no significant effect macrophages are multifunctional and able to on healing. These studies are some of the first to influence nearly all phases of repair. Modulation delineate the different roles of macrophages of macrophage function, then, is a logical and during the phases of the healing process and rapidly emerging target for wound therapeutics. support the concept that this cell exhibits different functional phenotypes as repair Role of macrophages in wound healing progresses. Landmark studies in the early 1970s and 1980s demonstrated that macrophages are critical to Recruitment of monocytes and wound healing, and the ability of macrophages macrophages at sites of injury to produce factors that stimulate angiogenesis Similarly to leukocyte migration at almost any site and fibroplasia has been firmly established of inflammation, monocyte recruitment into (Refs 9, 10, 11, 12). Early studies used guinea wounds involves the sequential steps of pigs depleted of macrophages by treatment endothelial cell activation, cell-to-cell interaction, with both antimacrophage antiserum and and transmigration through the endothelium Inflammation and wound healing: the role of the macrophage glucocorticoids to study the role of this cell into the extravascular space. Because monocytes in the healing wound (Ref. 9). Because represent only about 3% of circulating glucocorticoids have several additional effects leukocytes, the rate of monocyte influx into that might influence repair, these early wounds is far from stoichiometric. Initially, observations were limited in interpretation. monocytes may be recruited by factors Recent advances in the use of genetically produced quickly after injury, such as split modified mice have overcome this limitation. products from the coagulation cascade, factors These techniques allow a highly selective and released from platelet degranulation and specific depletion of macrophages in wounds activated complement components. But most and have confirmed a crucial role for monocyte infiltration occurs later, and the 2 Accession information: doi:10.1017/S1462399411001943; Vol. 13; e23; July 2011 © Cambridge University Press 2011

T cells

Macrophages

Neutrophils

Mast cells

Haemostasis Early inflammation Late inflammation Resolution/ remodelling

Time after injury

Figure 1. Pattern of leuKocyte infiltration into wounds. Inflammatory cells are present during each of the phases of wound repair, represented here as haemostasis (yellow panel), early inflammation (light pink panel), late inflammation (dark pink panel) and resolution/remodelling (blue panel). The relative density of the four most prominent types of leuKocytes in wounds (mast cells, neutrophils, macrophages and T cells) is depicted. Whereas neutrophils and lymphocytes disappear, low numbers of resident mast cells and macrophages are present during the lengthy remodelling phase.

preferential infiltration of monocytes represents (Refs 16, 17, 18, 19, 20). The role of chemokines the response to a locally produced chemotactic in the recruitment of leukocytes is complicated, gradient that specifically favours these cells. One because more than 40 chemokines have been important group of chemoattractants produced identified. The complexity of this process has within the wound are the chemokines, which been well described by others (Ref. 21). Inflammation and wound healing: the role of the macrophage are a group of related small proteins that display highly conserved cysteine amino acid Specific macrophage functions in wounds residues. Chemokines can be produced by many Promotion of inflammation cell types, and individual chemokines may Resting macrophages produce only low levels preferentially recruit particular populations. of proinflammatory mediators. On exposure Several studies have examined the expression to proinflammatory cytokines, interferons, and function of chemokines in healing wounds, lipopolysaccharides or other microbial products, and the general patterns of chemokine or DAMPs (such as heat-shock proteins, high- expression correlate with the movement of mobility group box proteins and molecular leukocytes, including monocytes, into wounds fragments of the extracellular matrix), 3 Accession information: doi:10.1017/S1462399411001943; Vol. 13; e23; July 2011 © Cambridge University Press 2011

macrophages acquire a proinflammatory or mentioned above, other factors, including IL-10, ‘classically activated’ phenotype (Ref. 22). glucocorticoids, prostaglandins, metabolites and Following activation, proinflammatory the process of efferocytosis (discussed in detail macrophages themselves produce a large number below), might also induce M2-like phenotypes. of mediators and cytokines, including interleukin In the context of the healing wound, the role of (IL)-1, IL-6, IL-12, tumour necrosis factor-α (TNF- each potential stimulus to the phenotypic switch α) and inducible nitric oxide synthase (Refs 23, and resolution of inflammation is not 24). Because many of these mediators have been completely understood. shown to be present in the early wound Until recently, the existence of discrete environment, macrophages seem to be a probable macrophage phenotypes within wounds was source (Ref. 23). Macrophages also produce largely assumed rather than proven. However, chemoattractants, including chemokines, that recent studies of macrophages derived from recruit additional leukocytes (Ref. 25). skin wounds, as well as sponges implanted subcutaneously in mice, demonstrate that Reparative and anti-inflammatory function macrophages do exhibit multiple phenotypes of wound macrophages that change over time (Ref. 29). These studies In vitro studies suggest that macrophages are suggest that M1-like cells, which are capable of transitioning from a proinflammatory characterized by the production of TNF-α, IL-1 to an ‘alternatively activated’, or reparative, and IL-6, are common in the early phases of phenotype (Refs 26, 27). The alternative repair, whereas M2-like cells, with less phenotype is characterised in part by expression proinflammatory cytokines and elevated of anti-inflammatory mediators such as IL-1 markers of alternative activation, including receptor antagonist, decoy IL-1 receptor type II CD206 and arginase 1, are common in the later and IL-10, and by the production of growth stages of repair. However, these in vivo factors such as TGF-β, VEGF and insulin-like phenotypes appear far from simple, and do not growth factor-1. The transition of macrophages to completely mimic the previously described in an alternative phenotype has been assumed to be vitro macrophage classifications. For example, requisite for the switch from inflammation to populations of macrophages were found to proliferation in the healing wound (Ref. 28). exhibit both markers of alternatively activated The canonical factors for inducing the macrophages, such as mannose receptors and alternative phenotype are IL-4 and IL-13. cytokines (TNF-α, IL-6) associated with a Curiously, however, recent studies suggest that classically activated phenotype. At any single IL-4 and IL-13 are not requisite for the time point, then, the wound bed may contain modulation of macrophage phenotypes in several discrete macrophage phenotypes or wounds in vivo (Ref. 29) Anti-inflammatory hybrid macrophage phenotypes. cytokines, glucocorticoids and modulators Several studies now suggest that alterations in of glucose and lipid metabolism induce a macrophage phenotypes have a critical role in the broad spectrum of ‘alternative’ macrophage pathogenesis of chronic wounds. For example, in phenotypes, including those that exhibit non- a murine wound model, iron overloading of or anti-inflammatory and pro-tissue repair macrophages has been shown to create functions. Recent studies suggest another dominant proinflammatory M1-like macrophage pathway in which M1 macrophages are induced populations, with resultant impaired wound to develop into a novel M2-like phenotype in a healing (Ref. 31). This study has significant Inflammation and wound healing: the role of the macrophage manner that is independent of IL-4 and IL-13 clinical implications because iron overload has (Ref. 30). In these in vitro studies, initial been previously associated with human chronic activation occurred by the engagement of toll- venous ulcers and iron-overloaded macrophages like receptors: an action that induces expression have been identified in chronic venous ulcers of the adenosine 2A receptor (A2AR). (Refs 31, 32). Thus, at least for venous ulcers, iron Subsequent interaction of A2AR with adenosine might create a persistant proinflammatory completes the activation, leading to an M2-like macrophage phenotype that is critical to the cell. This M2-like cell, dubbed the M2d failure to heal. In addition, our preliminary macrophage, has been suggested to be studies indicate that the transition from a important to wound-healing outcomes. As proinflammatory to a prohealing phenotype is 4 Accession information: doi:10.1017/S1462399411001943; Vol. 13; e23; July 2011 © Cambridge University Press 2011

impaired in excisional wound macrophages from Recent studies suggest that failure to remove diabetic db/db mice. However, the factors inflammatory cells, such as neutrophils, has a involved in maintaining the persistent role in the pathogenesis of nonhealing wounds proinflammatory wound macrophage phenotype (Refs 38, 44). A deficit in the capability of in diabetic mice remain to be elucidated. macrophages to effectively remove neutrophils has recently been reported to be a critical Removal of neutrophils and reduction of component of the impaired healing seen in apoptotic load in the wound diabetes (Ref. 38). Macrophages derived from One important function of wound macrophages sponges implanted in diabetic mice showed a is the capacity to facilitate the nonphlogistic significant impairment in the phagocytosis removal of neutrophils. Neutrophils are of apoptotic cells. This deficit was associated abundant in early wounds and are essential for with higher levels of apoptotic cells and effective decontamination. Yet a large body of proinflammatory mediators in wounds, a evidence suggests that neutrophils negatively feature that was further validated in wound influence repair, probably because this cell type tissues of diabetic patients. A deficit in is capable of destroying normal tissue (Ref. 33). macrophage phagocytic capability has also been Neutrophil proteases, such as elastase and associated with the delayed repair that occurs cathepsin G, can degrade most components of with ageing (Ref. 45). Successful efferocytosis by the extracellular matrix as well as proteins as macrophages therefore may be requisite for diverse as clotting factors, complement, appropriate wound healing, both for removal of immunoglobulins and cytokines (Refs 34, 35, apoptotic neutrophils and for the generation of a 36). Because the extracellular matrix serves as a macrophage phenotype that supports the supporting scaffold for infiltrating cells, proliferative aspects of repair. modification of the extracellular matrix by neutrophils could have important consequences Promotion of angiogenesis, fibroblast for repair. Neutrophils also produce a large proliferation and ECM synthesis number of free oxygen radicals, and thus are Macrophages influence wound healing through capable of inducing considerable oxidative stress the generation of growth factors that promote on the wound. Mediators such as superoxide cell proliferation and protein synthesis (Ref. 46), and hydrogen peroxide can cause additional as well as by the production of proteases and tissue damage, delaying the repair process and their inhibitors that influence ECM content and modifying healing outcomes (Ref. 37). The large remodelling. Several factors that are known to load of neutrophils is removed primarily by be present in healing wounds have been shown apoptosis. The removal of apoptotic cells by to be produced by macrophages (Refs 23, 24, phagocytosis, a process that is termed 47). In general, this information has been efferocytosis, prevents secondary necrosis of considered presumptive evidence for a these cells and is thought to be essential for macrophage-based influence on the healing complete repair (Ref. 38). process. However, direct evidence for the role of Macrophages are unique in wounds, because the macrophage as the critical source of these they represent the single most effective means of factors is difficult to obtain. Within the healing neutrophil removal. Macrophages assist in the wound, macrophages are rarely the sole source removal of neutrophils from sites of injuries in of any of these described factors, and many several ways. They respond to neutrophils and other cell types within the wound, including Inflammation and wound healing: the role of the macrophage their products, and can induce apoptosis in other immune cells, keratinocytes, fibroblasts, neutrophils (Ref. 39). Perhaps more importantly, endothelial cells and adipocytes, also produce macrophages recognise and actively ingest the same factors. apoptotic neutrophils, thus helping to resolve One excellent example of this conundrum is the wound inflammation (Refs 38, 40, 41, 42). capability of wound macrophages to influence Several studies suggest that the phagocytosis of angiogenesis by the production of VEGF. VEGF neutrophils influences macrophage phenotype, is a potent proangiogenic factor that has been causing a switch from proinflammatory to a shown to contribute 50% or more of the growth-promoting, reparative phenotype proangiogenic activity in wounds (Ref. 48). (Ref. 43). Macrophages certainly have proangiogenic 5 Accession information: doi:10.1017/S1462399411001943; Vol. 13; e23; July 2011 © Cambridge University Press 2011

capabilities, and are well documented to produce more recent study has demonstrated the abundant amounts of VEGF (Refs 11, 48, 49). simultaneous tracking of neutrophils and However, in epithelial wounds, keratinocytes macrophages labelled with differential also produce plentiful amounts of VEGF, fluorescent labels into a tail fin injury site making it difficult to determine the relative (Ref. 55). In this study, the two cell types contribution of macrophages versus showed marked differences in migration speed keratinocytes in dictating the angiogenic and kinetics of recruitment to the injury site. phenotype (Ref. 50). A definitive answer to the Notably, the study revealed a preferred pathway question of the importance of macrophage for macrophages along the abluminal surface of VEGF during wound healing would require the endothelial cells. The powerful approaches selective temporal depletion of VEGF from available in the zebrafish, including tracking of the wound macrophage. Such experiments are several inflammatory cell types, specific ablation increasing in feasibility because of the of cell types, and the ability to perform development of genetic mutants with selective mutagenesis and transgenesis, suggest that deficiencies. A recent study using mice with a many difficult questions about macrophage deletion of VEGF solely from cells of myeloid function in wounds can be approached in this origin demonstrated that this deficiency yields model system. delayed excisional wound healing, with little impact on incisional healing (Ref. 51). Clinical implications Another caveat to the interpretation of the role of Improving macrophage function to improve macrophage-derived factors is that many are healing outcomes known to have both direct and indirect effects An estimated 6 million people in the US have on repair outcomes. For example, PDGF from problems related to inadequate wound healing, wound macrophages might assist in the and nonhealing ulcers remain a serious problem recruitment of progenitor cells and additional that greatly affects human health (Ref. 56). The inflammatory cells (Ref. 52). More recently, improvement of wound healing therefore PDGF has also been shown to cause fibroblasts continues to be the target of many therapeutic to produce osteopontin, a factor that critically strategies. Many attempts to augment the influences wound healing through an autocrine healing process have used single growth factors effect that promotes scar formation (Ref. 53). or cytokines, mostly with limited success. When A full understanding of the complex role of using single factors, difficulties with optimum macrophage-derived factors is likely to benefit delivery systems, timing and concentration are from emerging technologies. Future investigations daunting tasks. In addition, the chronic wound to untangle the web of macrophage-derived environment can be highly proteolytic, limiting factors might include global descriptions of the half-life of topically applied molecular factors. macrophage mediator production patterns in One alternative to the use of molecular healing wounds, along with analysis of direct and therapeutics for wounds is in situ activation, indirect effects of macrophage products in vivo recruitment or addition of exogenous and in vitro. Such large data sets, once generated, macrophages. Because macrophages are a source might benefit from advanced biostatistical analysis of growth factors, augmented macrophage in order to develop models to explain these activity may stimulate cellular proliferation and complex interactions. angiogenesis in nonhealing wounds. Increasing One approach to the study of wound repair that the number of macrophages in the wound might Inflammation and wound healing: the role of the macrophage continues to provide relevant information is the also influence the protease imbalance that occurs use of genetically tractable organisms, such as in some nonhealing wounds, because zebrafish and Drosophila. Zebrafish, owing to macrophages can produce protease inhibitors. their near transparence, provide the additional Finally, the addition of more macrophages might advantage of allowing real-time live imaging of provide an increased efferocytosis capacity. leukocyte infiltration into sites of inflammation In support of the therapeutic potential for and injury. Previous studies have examined increased macrophage activity in the healing macrophage infiltration into zebrafish wounds wound, several early studies document that the and have documented some of the cytoskeletal topical treatment of wounds with the requirements for this migration (Ref. 54). A macrophage-activating agent glucan (Refs 57, 58) 6 Accession information: doi:10.1017/S1462399411001943; Vol. 13; e23; July 2011 © Cambridge University Press 2011

improves healing outcomes. Glucans are polymers that early fetal wounds exhibit very little, if any, of β-1,3-linked glucose that interact with inflammatory response while healing in a scarless polysaccharide receptors on the macrophage, fashion (Refs 7, 63). Intrinsic differences in causing cell activation. Glucan treatment of growth factor production, levels of stem cells and wounds has been shown to increase the number cellular proliferation capacity probably support of macrophages, and promote fibroplasia, re- fetal wound repair. However, the observed epithelialisation and wound strength. Similarly, scarless repair in the absence of inflammation the application of chemoattractants such as suggests that macrophages are not an essential MCP-1 that recruit monocytes to wounds has feature of adequate repair, particularly if other been shown to promote healing (Ref. 59). aspects of the inflammatory response are Conceptually, the supplementation of wounds suppressed. Several recent studies on mice with exogenous macrophages could promote genetically deficient in specific immune cells and repair, particularly when macrophage function molecules also support the concept that is deficient. Compelling evidence in favour of inflammatory cells are not needed for efficient such a strategy comes from Danon and tissue repair, as long as microbial contamination –/– colleagues (Ref. 60), who demonstrated that the is controlled. In neonatal Sfp1 mice (previously –/– injection of macrophages into the wounds of known as PU.1 ), which lack both macrophages aged mice could correct the age-related deficit in and functioning neutrophils, little inflammation wound healing. Studies in human subjects have occurs at the wound site, and repair appears to shown that monocyte-derived macrophages, be scar free, similarly to that in the fetus (Ref. 64). obtained from peripheral blood, can improve In addition, mice with a deletion of the gene the healing of pressure ulcers, as well as sternal encoding Smad3, a molecule critical to the chest wounds, in patients undergoing cardiac intracellular signalling of TGF-β1, exhibited surgery (Refs 61, 62). accelerated cutaneous wound healing and Taken together, these findings suggest that significantly reduced influx of inflammatory cells therapeutic strategies that increase macrophage (Ref. 65). Likewise, studies in a number of other accumulation could accelerate the wound- systems have shown that soluble factors that healing process, and might be particularly reduce inflammation in wounds, such as the helpful in situations of impaired healing, such as cytokine IP-10 and heparin-binding epidermal ageing and diabetes. In particular, agents that growth factor, are often beneficial to wound- could recruit and drive macrophages towards a healing outcomes in the adult organism (Refs 66, reparative phenotype would promote tissue 67). Together, these findings suggest that in the regeneration in the absence of destructive face of normal inflammation, which includes inflammation. The clinical implications of oedema, mast cell degranulation and neutrophil adding such a tool to the therapeutic arsenal ingress, macrophages have an important could be great. As mentioned above, nonhealing balancing role. By contrast, when the complete wounds are an immense problem, and current inflammatory response is severely suppressed treatments are frequently ineffective. Moreover, and bacterial contamination is controlled, evidence indicates that macrophage dysfunction wounds appear to heal well. Thus, the role of has a role in the impaired healing seen in macrophages must always be considered in the diabetic patients and in the elderly, who are at context of the specific wound environment in great risk for the development of nonhealing question. Reductionist approaches have provided ulcers (Refs 38, 45). a wealth of information about the function of Inflammation and wound healing: the role of the macrophage wound macrophages; however, studies that focus Research in progress and outstanding on the integration of macrophage function with research questions those of the many other cell types in the wound In the absence of other inflammatory cells, are required. are macrophages essential for healing? Although the studies described above suggest that Macrophage phenotypes in the healing macrophages are generally beneficial to repair, wound some controversy remains over the role of As mentioned above, abundant in vitro studies macrophages in the healing wound. Studies of suggest that macrophages can adopt discrete healing in the early- to mid-gestation fetus show phenotypes in response to environmental 7 Accession information: doi:10.1017/S1462399411001943; Vol. 13; e23; July 2011 © Cambridge University Press 2011

signals (Ref. 68). By contrast, there are few studies from excisional wounds in normally healing mice of the actual macrophage phenotypes that exist in shows that wound macrophages exhibit high- the in vivo wound and the phenotypes described level Ly6C expression early following wounding, so far appear highly complex (Ref. 29). Moreover, which is associated with a proinflammatory several markers of macrophage phenotypes have phenotype. As healing progresses, wound been proposed, and there is a lack of agreement macrophages exhibit a transition to low-level over which are the most critical or informative. Ly6C expression, which is associated with a Finally, investigations of wound macrophage prohealing phenotype. Whether the observed phenotypes are limited by technical difficulties phenotype transition is derived from an in situ in obtaining truly representative sample response to differential environmental cues or populations of cells from specific locations in from selective recruitment of monocyte wounds. populations that are already predisposed to Relevant to the question of wound macrophage proinflammatory or prohealing phenotypes phenotypes is the question of the origin of wound remains to be determined. macrophages. Most wound macrophages are Another question is the role of macrophages in thought to be derived from circulating mediating the resolution phase of healing. During monocytes. These cells in mice exist in two main this final phase, capillary regression and collagen subsets: an ‘inflammatory’ subset expresses Ly6C remodelling are dominant features. Macrophages at high levels and a ‘noninflammatory’ subset can produce factors that are antiangiogenic (such expresses Ly6C at low levels (Refs 69, 70). as thrombospondin-1 and IP-10) (Refs 72, 73) and Recently, similar populations of proinflammatory others, including CXCR3 ligands, that direct the CD14lo and noninflammatory CD14hi monocytes termination of the repair response in several have been described in humans (Ref. 71). Our ways (Refs 20, 74). Other cells within the wound preliminary research on macrophages isolated may also produce these concluding signals, but

Activation Phagocytosis Phagocytosis of neutrophils Pro-inflammatory Fibrotic and angiogenic factors factors Anti-fibrotic, Induction of neutrophil Anti-inflammatory anti-angiogenic apoptosis factors factors

Early Intermediate Late Very late (?) phenotypes? Inflammation and wound healing: the role of the macrophage Likely pattern of macrophage function during the course of wound healing Expert Reviews in Molecular Medicine © 2011 Cambridge University Press

Figure 2. Likely pattern of macrophage function during the course of wound healing. In the early wound, monocytes and resident macrophages become activated, undertake phagocytosis of microbes and perhaps early neutrophils, and produce proinflammatory mediators and chemoattractants. Macrophages also assist in the induction of apoptosis in neutrophils, thus steering the wound towards a noninflammatory, reparative state. In the later phases of wound repair, macrophages ingest apoptotic neutrophils, producing growth factors to support tissue restoration. In the very late stages, as the wound resolves, macrophages may guide tissue remodelling by producing factors to promote capillary regression and collagen remodelling. 8 Accession information: doi:10.1017/S1462399411001943; Vol. 13; e23; July 2011 © Cambridge University Press 2011

because macrophages can theoretically be a major 8 Martin, P. and Leibovich, S.J. (2005) Inflammatory source of these types of factors, these cells might cells during wound repair: the good, the bad and the have an active role in the termination of the ugly. Trends in Cell Biology 15, 599-607 wound-healing process. Little is known about 9 Leibovich, S.J. and Ross, R. (1975) The role of the how macrophages might shut down the healing macrophage in wound repair. A study with response, and much remains to be learned about hydrocortisone and antimacrophage serum. this phase. American Journal of Pathology 78, 71-100 A composite of macrophage functions 10 Polverini, P.J. et al. (1977) Activated throughout the time course of the healing wound macrophages induce vascular proliferation. is suggested from the current literature (Fig. 2). Nature 269, 804-806 Whether wound macrophages fall into specific 11 Hunt, T.K. et al. (1984) Studies on inflammation and discrete phenotypes remains to be determined, yet wound healing: angiogenesis and collagen synthesis the significance of gaining a true understanding of stimulated in vivo by resident and activated wound macrophage phenotypes within wounds could be macrophages. Surgery 96, 48-54 huge. Macrophages are probably important 12 Kovacs, E.J. and DiPietro, L.A. (1994) Fibrogenic contributors to pathophysiology in nonhealing cytokines and connective tissue production. FASEB wounds, delayed healing, and fibrosis and scar Journal 8, 854-861 formation, thus the macrophage remains an 13 Goren, I. et al. (2009) A transgenic mouse model of attractive target for therapeutic strategies. inducible macrophage depletion: effects of diphtheria toxin-driven lysozyme M-specific cell Acknowledgements and funding lineage ablation on wound inflammatory, Wound-healing research in L.A.D.’s laboratory is angiogenic, and contractive processes. American supported by National Institute of Health Journal of Pathology 175, 132-147 Grants RO1-GM50875 and P20-GM078426. 14 Mirza, R., DiPietro, L.A. and Koh, T.J.(2009) Selective Tissue repair research in T.J.K.’s laboratory is and specific macrophage ablation is detrimental to supported by the United States Army Medical wound healing in mice. American Journal of Research and Materiel Command #W81XWH- Pathology 175, 2454-2462 05–1-0159. The authors thank the reviewers for 15 Lucas, T. et al. (2010) Differential roles of their critical comments and suggestions. macrophages in diverse phases of skin repair. Journal of Immunology 184, 3964-3977 References 16 Fahey, T.J. 3rd et al. (1990) Cytokine production in a 1 Zhang, X. and Mosser, D.M. (2008) Macrophage model of wound healing: the appearance of activation by endogenous danger signals. Journal of MIP-1, MIP-2, cachectin/TNF and IL-1. Cytokine 2, Pathology 214, 161-178 92-99 2 Sen, C.K. (2009) Wound healing essentials: let there 17 DiPietro, L.A. et al. (1995) Modulation of JE/MCP-1 be oxygen. Wound Repair and Regeneration 17, 1-18 expression in dermal wound repair. American 3 Ross, R. and Odland, G. (1968) Human wound Journal of Pathology 146, 868-875 repair. II. Inflammatory cells, epithelial- 18 DiPietro, L.A. et al. (1998) MIP-1alpha as a mesenchymal interrelations, and fibrogenesis. critical macrophage chemoattractant in murine Journal of Cell Biology 39, 152-168 wound repair. Journal of Clinical Investigation 101, 4 Artuc, M. et al. (1999) Mast cells and their mediators 1693-1698 in cutaneous wound healing – active participants or 19 Wetzler, C. et al. (2000) Large and sustained innocent bystanders? Experimental Dermatology induction of chemokines during impaired wound Inflammation and wound healing: the role of the macrophage 8, 1-16 healing in the genetically diabetic mouse: prolonged 5 Barbul, A. et al. (1989) Wound healing in nude mice: persistence of neutrophils and macrophages during a study on the regulatory role of lymphocytes in the late phase of repair. Journal of Investigative fibroplasia. Surgery 105, 764-769 Dermatology 115, 245-253 6 Barbul, A. et al. (1989) The effect of in vivo T helper 20 Engelhardt, E. et al. (1998) Chemokines IL-8, and T suppressor lymphocyte depletion on wound GROalpha, MCP-1, IP-10, and Mig are sequentially healing. Annals of Surgery 209, 479-483 and differentially expressed during phase-specific 7 Cowin, A.J. et al. (1998) Endogenous inflammatory infiltration of leukocyte subsets in human wound response to dermal wound healing in the fetal and healing. American Journal of Pathology 153, adult mouse. Developmental Dynamics 212, 385-393 1849-1860 9 Accession information: doi:10.1017/S1462399411001943; Vol. 13; e23; July 2011 © Cambridge University Press 2011

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Further reading, and resources and contacts

Barrientos, S. et al. (2008) Growth factors and cytokines in wound healing. Wound Repair and Regeneration 16, 585-601 This review examines the specific roles of these growth factors and cytokines during the wound-healing process. Brancato, S.K. and Albina, J.E. (2011) Wound macrophages as key regulators of repair origin, phenotype, and function. American Journal of Pathology 178, 19-25 This article focuses on the phenotype of wound macrophages, and the functions attributed to the role of macrophages in healing wounds. Wilgus, T.A. (2008) Immune cells in the healing skin wound: influential players at each stage of repair. Pharmcological Research 58, 112-116 This article explains how different immune cell lineages function in the various stages of repair.

Features associated with this article

Figures Figure 1. Pattern of leuKocyte infiltration into wounds. Figure 2. Likely pattern of macrophage function during the course of wound healing.

Citation details for this article

Timothy J. Koh and Luisa Ann DiPietro (2011) Inflammation and wound healing: the role of the macrophage. Expert Rev. Mol. Med. Vol. 13, e23, July 2011, doi:10.1017/S1462399411001943 Inflammation and wound healing: the role of the macrophage